Principles of Cathodic Protection and Its Application to Steel in Concrete

Principles of Cathodic Protection and Its Application to Steel in Concrete

Steel in concrete is usually protected against corrosion by passivation of the steel arising from the high alkalinity of the pore solutions within the concrete. A stable oxide layer is formed on the steel surface which prevents the anodic dissolution of iron. The necessity for additional protective measures arises if this stable oxide layer is rendered unstable (if depassivation occurs) due to the ingress of chlorides to the steel/concrete interface or carbonation of the concrete reducing the alkalinity of the pore solution at the steel/concrete interface.

In the case of chloride contamination of concrete, the chloride ions initiate depassivation which leads to corrosion if there is access of oxygen to the remaining passive areas. Depassivation and, hence, corrosion can be obtained by the establishment of a specific steel/concrete potential, the pitting potential Epit. At potentials more positive than Epit, a sharp increase in the iron dissolution rate leads to high corrosion rates in small localized areas of the steel surface whereas, at lower potentials than Epit (i.e. more negative), the corrosion rate decreases. The objective of cathodic protection is to shift the steel/concrete potential into a region where

a) the initiation of corrosion, or
b) if corrosion has already started, the continuation/propagation of corrosion; is so far suppressed that a corrosion failure is unlikely during the lifetime of the structure.

In the case of reinforced concrete, a corrosion failure can include cracking and delamination of the covering concrete which may arise from as little as 50 μm of metal loss from an area of reinforcement or other embedded steel, due to the bursting stresses generated by high-volume corrosion products.

In steel-reinforced concrete structures, cathodic protection can be achieved by polarizing the reinforcement/steel with an “external” current. For this purpose, anodes are surface mounted, painted on to or embedded in the concrete and connected to the positive pole of a direct current power supply in the case of impressed current protection. Alternatively, anodes of zinc or Al-Zn-In are applied to the concrete and connected directly to the reinforcement.

The system cathode is formed by the steel reinforcement/steel. In the case of impressed current, the negative pole of the direct current power supply is connected to the embedded steel/reinforcement. In the case of galvanic anode cathodic protection, the galvanic anode (typically zinc) is connected directly to the reinforcement/steel.

The concrete, in particular the pore solution, provides the electrolyte to allow current flow and the associated ionic movement. The change of steel/concrete potential is indicated by electrodes which are embedded in the concrete or placed on the surface of the concrete and used, in conjunction with suitable instrumentation and connections to the reinforcement/steel, to measure steel/concrete/electrode potentials.